The present invention relates to a disk chipper for the production of wood chips.
The wood chips used as raw material in the production of chemical pulp and TMP (Thermo-Mechanical Pulp) are produced by chipping round wood by means of a chipper. Among the previously known types of chipper, the so-called disk chipper is the most commonly used.
The basic structure of the disk chipper consists of a rotating chipping disk mounted on a horizontal or slightly inclined axle supported by bearings. The chipping disk is typically a steel plate rotating in a vertical or nearly vertical position. The disk is provided with radial or nearly radial holes and chipping blades are attached to the rear edges of these holes, as seen in the direction of rotation. This disk and its axle are supported by bearings on a fixed frame. The frame is also provided with a feed mouth for feeding the logs onto the chipping disk. At the bottom of the feed mouth, at a position closest to the chipping disk, is a counter blade against which the log rests while chipped by the blades in the chipping disk.
The chipping disk is rotated by means of a drive machine. This may be a synchronous motor connected to the axle by a coupling, in which case the disk has the same rotational speed as the motor. Another commonly used arrangement is one in which the motor is connected to the primary shaft of a gear box connected to the chipper axle by a coupling. In this arrangement, the motor rotates at a higher speed than the disk. In another previously known disk drive, the gear box has two primary shafts driven by respective driving motors.
The amount of energy required for the chipping depends on the strength of the wood and is therefore different for different varieties of wood. Typical characteristic chipping energy levels for different types of wood variety are as follows:
______________________________________ coniferous wood 6000-8000 kJ/M.sup.3 s.m. hardwood 8000-10000 kJ/m.sup.3 s.m. tropical special wood varieties 10000-12000 kJ/m.sup.3 s.m. ______________________________________
where m.sup.3 s.m. means solid meters.
When the wood variety to be chipped is known, and the capacity of the chipping line has been determined, the power required by the chipper under normal circumstances can be calculated by the equation: EQU P = Q * p
where
Q = capacity [m.sup.3 s.m.] PA0 p = characteristic chipping energy [kJ/m.sup.3 s.m.] PA0 D = diameter of log [m] PA0 1= chip length [m] PA0 Z = number of chipping blades PA0 n = speed of rotation of the chipping disk [r/min] PA0 D =800 mm (coniferous wood) PA0 1=22 mm PA0 Z =12 pcs PA0 n =240 r/min PA0 P =583 kW PA0 P.sub.max =3715 kW PA0 M.sub.max =147.84 kNm
The maximum power required by the chipper occurs when a log of the maximum size is fed into it. The maximum power requirement can be calculated as follows: EQU p.sub.max =(D/2).sup.2 * .pi.* 1 * Z * n * 60 * p
where
The chipping line capacity Q typically varies between 200-300 m.sup.3 s.m./h, for which the average power requirement is in the range of 390-680 kW.
Similarly, the maximum log diameter acceptable to the chipper is D =800-900 mm. The number of chipping blades commonly used is 12 and the speed of rotation of the chipping disk 240-300 r/min. This means that the maximum power requirement in the case of coniferous wood and 24 mm chip length is 4050-6415 kW.
From this it can be seen that the maximum power requirement is nearly ten times that of the normal input power. In normal use, such high power levels are extremely seldom needed. The average diameter of logs used in the production of wood pulp varies from plant to plant between D =100-250 mm. However, as the chipper must be able to handle even the occasional very large logs, the installed power of the chipper is determined by the maximum log size. The decisive factor in determining the power requirement is expressly the high torque required by the maximum log size, and the drive machine must be capable of generating a corresponding torque.
The motor to be used with a chipper is selected by determining the torque required by the maximum log size. On the basis of this torque, the drive motor is so selected that the break-down torque of the motor is higher than the required torque.
For example, if:
and the chipping line capacity is assumed to be Q =300 m.sup.3 s.m./h, then
Choosing a gear with a transmission ratio of 6.25, which is advantageous because it allows a motor with the standard rotational speed of 1500 r/min to be used, means that the motor must be able to generate a torque of: ##EQU1##
If a gear box with two primary shafts driven by respective motors is selected, then each motor must be able to generate a torque of EQU M/hd m/2=11827 Nm
This corresponds to a maximum motor power of ##EQU2##
If the motor has a break-down torque factor of 2, then when standard motors are selected: EQU P.sub.n =1000 kW (for each of 2 motors)
Since the selection of motor size is based solely on the break-down torque produced by the motor, the break-down torque factor of the motor plays an essential role in the determination of motor size.
Because of the questions of compatibility and spare parts, standard motors are generally preferred. In the standard motor series, motors in the lower power range (250-450 kW) are designed for higher break-down torque factors than those in the highest power range (500-1000 kW). This means that motors in the higher power range cannot be used as efficiently as those of the lower power range, thereby requiring a substantial overcapacity in terms of the installed power. Additionally, high power motors require very high peak currents during start-up, thereby demanding the use of large, and expensive, power supplies. Furthermore, in order to reduce peak currents during start-up, high power motors are typically coupled to the disk shaft through a fluid coupling. Hydraulic losses in the fluid coupling substantially reduces the torque actually transmitted to the disk shaft during operation, further reducing the efficiency of the installation.